The Motion Pictures Expert Group (MPEG) standard authorized by the International Organization for Standardization (ISO) and International Telecommunication Organization (ITO) is widely used for processing digital video image data. Base on the MPEG standard, video image data can be compressed to be a sequence of frames of three different types, i.e. Intra (I) frames, Predictive (P) frames, and Bi-directionally Predictive (B) frames. The I-frames are independently coded and are the least efficiently coded of the three frame types. The P-frames are coded relative to the previously coded I- or P-frame such that only the differences between the P-frame and the previously coded I- or P-frame are stored. The B-frames are coded relative to both the previous and the next I- or P-frames so that only the differences between the B-frame and the previous and next I- or P-frames are captured, compressed, and stored. In this manner, the data amounts required to represent digital video images are reduced.
When decoding the compressed video images, the I-frames can be decoded independently. However, the P-frames must be decoded based on the previously decoded I- or P-frame and the B-frames must be decoded based on the previously and the next decoded I- or P-frames. Because of the frame dependencies, the order in which the frames are decoded or stored is not necessarily the same as the order in which the frames are displayed. For example, the frame sequence in display order can be:    I1 B1 B2 P1 B3 B4 P2 B5 B6 P3 B7 B8 I2 B9 B10 P4 B11 B12 P5 B13 B14 P6 However, the frame sequence in decoded/stored order is different from the display order, and can be:    I1 P1 B1 B2 P2 B3 B4 P3 B5 B6 I2 B7 B8 P4 B9 B10 P5 B11 B12 P6 B13 B14 
It is also because of the frame dependencies that a memory device, i.e., frame buffer, is needed to store the decoded I-frames, P-frames, and B-frames when decoding the video image data that is compressed based on the MPEG standard. With the reference to the exemplary frame sequence in decoded/stored order shown above, the I-frame and the P-frame, such as I1 and P1, must be decoded first and then the two B-frames corresponding to the previously decoded I- and P-frame, such as B1 and B2, are decoded so that the B-frames can be decoded with the reference of the previously decoded I-frame and P-frame. The decoded I-frames, P-frames, and B-frames are stored in specific locations of the frame buffer. The capacity of the frame buffer is a major cost in the production of a video decoder. Thus, it is desired to reduce the required capacity of the frame buffer in order to reduce its cost.
The video image data can be in either a progressive or an interlaced format. The progressive format is a frame-based data structure and the interlaced format is a field-based data structure. Each frame in an interlaced format can be divided into two fields, referred to as top and bottom fields. The top field contains every other line in the frame beginning with the first line. The bottom field contains every other line beginning with the second line. In other words, the top field comprises the odd lines and the bottom field comprises the even lines. The interlaced video image data is processed and displayed in a field-based manner.
Most video image data formats are compatible with either the NTSC (National Television Standard Committee) or the PAL (Phase Alternating Line) standard. The NTSC format is 720×480 pixels per frame and the PAL format is 720×576 pixels per frame. The frame rate is 30 frames per second under the NTSC standard and 25 frames per second under the PAL standard.
There is thus a general need in the art for method and apparatus for decoding compressed video image data, which at least include a frame buffer with reduced size so that the cost can be reduced.